Nonaqueous secondary batteries such as lithium ion secondary battery (LIB) have already been prevailing widely, mainly in portable equipment applications. In the future, it is expected that nonaqueous secondary batteries will be rapidly enlarged in in-vehicle applications such as electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and the like. While promoting higher capacity, higher rate characteristics and upsizing in the development of in-vehicle LIBs, higher safety is required more than before. Accordingly, a separator is also required to have excellent ion permeability which enables permeation of lithium ions with uniform and low resistance, and safety such as heat resistance or short-circuiting resistance.
To cope with high-level demands for heat resistance among safety, a separator including a heat-resistant protective layer (HRL) provided on one or both surface(s) of a polyolefin porous film is disclosed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2008-243805 and Japanese Unexamined Patent Publication (Kokai) No. 2010-92881). However, because of limitative effects of HRL, a large-sized battery may be unable to effectively suppress shrinkage of the separator when meltdown of the polyolefin layer occurs over a large area, thus causing short-circuiting, especially at the end. Furthermore, it is usually difficult to form into a thin film since the separator is in the form of a laminate.
Thus, there is a need to use a material having excellent heat resistance as the separator to enhance safety of the battery. For example, Japanese Unexamined Patent Publication (Kokai) No. 5-335005, Japanese Unexamined Patent Publication (Kokai) No. 2005-209989 and Japanese Unexamined Patent Publication (Kokai) No. 2001-98106 disclose that an aromatic polyamide (aramid) having excellent heat resistance and oxidation resistance is used alone as a separator. Japanese Unexamined Patent Publication (Kokai) No. 5-335005 is an example which discloses applications of an aramid nonwoven fabric or an aramid paper as the separator. However, when a nonwoven fabric or a paper-like sheet is allowed to have a thickness of 50 μm or less, sufficient mechanical strength is not attained and it is difficult to industrially produce those having uniform and fine gap between fibers, thus causing fears that short-circuiting might occur. Meanwhile, Japanese Unexamined Patent Publication (Kokai) No. 2005-209989 is an example which discloses an aramid porous film formed by using a so-called “wet type” method in which an aramid solution is cast, followed by immersion in a solidification bath, thus causing precipitation. However, in the wet type method, a coating layer is formed on a surface, and a structure with numerous septa between pores and an independent pore are formed, and also the shape of the pore varies depending on a thickness direction of the film, thus making it difficult to control a pore structure. Whereas the wet type method having such problems, Japanese Unexamined Patent Publication (Kokai) No. 2001-98106 discloses a method in which an aramid film containing metal oxide fine particles dispersed therein is formed, and then the fine particles are dissolved and removed to obtain a porous film containing uniform pores. However, the porous film obtained by that method is a porous film in which the portion connecting between pores formed as a result of coming off of fine particles is a fine gap and this connection portion can serve as a bottle neck. In addition, remaining fine particles may come off when used as a separator.
It could therefore be helpful to provide a porous film which ensures high rate characteristics and is also capable of suppressing deterioration of cycle characteristics and storage characteristics as well as abnormal heat generation, when used as a separator for battery, and also has excellent heat resistance; and a separator for battery and a battery, each including the porous film.